Resistance spot welding device

ABSTRACT

A resistance spot welding device for welding at least two overlapping steel sheets held between a pair of welding electrodes is provided. The resistance spot welding device includes the pair of electrodes, an electrode force gauge that measures an electrode force, and a controller that controls an electric current supply to the electrodes according to the electrode force measured by the electrode force gauge. The controller controls the electric current such that the electrode force F measured by the electrode force gauge after the start of the electric current supply is adjusted to a prescribed value.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2016/001174, filedMar. 3, 2016, which claims priority to Japanese Patent Application No.2015-043053, filed Mar. 5, 2015, and Japanese Patent Application No.2015-071650, filed Mar. 31, 2015, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a resistance spot welding device forresistance spot welding, which is one type of lap resistance welding.

BACKGROUND OF THE INVENTION

To achieve an improvement in reliability of vehicle bodies and areduction in weight of the vehicle bodies for the purpose of animprovement in fuel consumption, strengthening of steel sheets has beenpursued in recent years. The use of high-strength steel sheets allowsvehicle bodies to have stiffness comparable to that obtained usingconventional steel even when the steel sheets are reduced in thicknessand weight. However, some problems have been pointed out. One of them isa reduction in the strength of welded portions.

As shown in FIG. 1, in resistance spot welding, a sheet set 3 includingat least two overlapping steel sheets (in this case, two steel sheetsincluding a lower steel sheet 1 and an upper steel sheet 2) is heldbetween a vertical pair of electrodes (a lower electrode 4 and an upperelectrode 5), and an electric current is applied under pressure to melta contact region between the steel sheets 1 and 2. A nugget 9 of therequired size is thereby formed, and a weld joint is obtained.

The quality of the joint obtained in the manner described above isevaluated by, for example, the diameter of the nugget, tensile shearstrength (the strength when a tensile test is performed in a sheardirection of the joint), cross-tension strength (the strength when atensile test is performed in a separation direction of the joint), orfatigue strength. In particular, as the strength of a steel sheetincreases, the amount of C in the steel sheet tends to increase. It isknown that, in a high-strength steel sheet containing a large amount ofC, its cross-tension strength is low.

In terms of the welding method, to ensure the cross-tension strengthwhen the high-strength steel sheet is used, it is contemplated toincrease the diameter of the nugget. Generally, to increase the size ofthe nugget, an electric current must be increased. In this case, thepossibility of the occurrence of expulsion becomes high. If expulsionoccurs, the nugget is rather reduced in size, and this causes areduction in the strength of the joint.

In particular, the surfaces of steel sheets for automobiles aresubjected to galvanization treatment with zinc as a main component forthe purpose of rust prevention. It is known that, when steel sheetshaving such galvanized layers are used for automobiles and subjected toresistance spot welding to assemble the automobiles, expulsion is likelyto occur, and it is therefore difficult to ensure large nuggets.

A conventional technique disclosed in Patent Literature 1 is a methodfor forming nuggets in three stacked steel sheets. In this method, aftera first welding step is performed, second and subsequent welding stepsare performed such that the supply of electric current and thesuspension of the supply are repeated in a pulsated manner. This allowsnuggets with sufficient diameters to be formed even when the sheet setincluding three sheets is composed of a thin sheet, a thick sheet, and athick sheet.

It is stated in Patent Literature 2 that, when steel sheets having ontheir surfaces alloyed aluminum coating layers containing Fe at anatomic percentage of from 50% to 80% inclusive are welded, a nugget canbe formed stably by specifying, according to the thickness of thesheets, the period of time during which the current is held constantafter upslope energization.

It is stated in Patent Literature 3 that, with zinc or zinc alloy coatedsteel sheets, a nugget having a certain size can be ensured by limitingthe ratio of the period of preliminary energization to the period of theformation of the nugget.

It is stated in Patent Literature 4 that, with zinc or zinc alloy coatedsteel sheets, a nugget having a certain size can be ensured by, afterpreliminary energization, repeating cooling and energization using acurrent value higher than the current value for the preliminaryenergization.

PATENT LITERATURE

PTL 1: Japanese Patent No. 4728926

PTL 2: Japanese Unexamined Patent Application Publication No.2011-167742

PTL 3: Japanese Patent No. 3849539

PTL 4: Japanese Patent No. 3922263

SUMMARY OF THE INVENTION

However, the methods described in Patent Literatures 1 to 4 still have aproblem in that it is difficult to ensure a nugget diameter stably. Inparticular, at a welding site for actual automobile assembly, unintendedwork disturbances such as a sheet gap are present and affect theformation of a nugget. With the methods described in Patent Literatures1 to 4, it is further difficult to ensure a nugget diameter stably whenwork disturbances are present.

In certain embodiments of the present invention a resistance spotwelding device that can form a large nugget while the occurrence ofexpulsion is prevented, whereby the diameter of the nugget can beensured stably even when work disturbances such as a sheet gap arepresent, is provided.

Resistance spot welding of a sheet set including high-strength steelsheets was repeatedly studied. In the studies, the electrode forceapplied between the electrodes during welding was measured in real time.Specifically, a pressure mechanism for applying pressure to the upperelectrode was used to measure the value of the load applied between theelectrodes during welding in real time (the load value is referred to asa measured electrode force or simply as an electrode force). Theelectrode force applied through the upper electrode can be set to adesignated value (referred to as a designated electrode force). However,the measurement results showed that the designated electrode forcediffered from the electrode force applied between the electrodes duringwelding.

It was also determined that there is a close relation between theoccurrence of expulsion and the electrode force. Specifically, in thecase where the electrode force increased rapidly in an initial stage ofthe electric current supply, expulsion occurred when the electrode forceexceeded a certain value. Then, after the electric current supply wassuspended and the electrode force was reduced, the electric currentsupply was resumed. In this case, no expulsion occurred even when theelectrode force was higher than that in the first electric currentsupply.

Examples of the results obtained by are shown FIGS. 2 and 3. FIG. 2 is agraph showing changes in electrode force with respect to the initialelectrode force when the resistance spot welding was performed while theelectric current supply and the suspension of the supply were repeated.FIG. 3 is a graph showing changes in electrode force with respect to theinitial electrode force when the resistance spot welding was performedwhile a constant electric current was supplied. In these experiments, apressure was applied for about 10 cycles (200 ms) before the electriccurrent was supplied to thereby obtain a stable state, and then theelectric current supply was started. The average electrode force in thefirst cycle (20 ms) after the start of the electric current supply isreferred to as the initial electrode force.

As shown in FIG. 2, no expulsion occurred when the electric currentsupply and the suspension of the supply were repeated, and the diameterof the nugget obtained was large. However, as shown in FIG. 3, when aconstant electric current was supplied continuously, expulsion occurred,and the diameter of the nugget was small.

Without being bound by theory, this mechanism may be as follows. In theinitial stage of the electric current supply, a nugget is formed rapidlyand expands, and this causes the measured electrode force to increase.Expulsion may occur when the pressurized state around the nugget isinsufficient. Specifically, when the measured electrode force becomesequal to or larger than a prescribed value during the electric currentsupply, the relative pressure around the nugget decreases, and this maycause expulsion to occur. When the increase in the electrode force issmall, which indicates the thermal expansion due to the formation of thenugget is small, and this results in an insufficient nugget diameter.

When the electric current supply is suspended, the nugget solidifies andcontracts, and the measured electrode force decreases. At the same time,heat is transferred to the surroundings of the nugget, and thetemperature of the portion around the nugget increases. This portion isthereby softened, and the pressurized state by the electrodes isensured, so that the occurrence of expulsion can be prevented. However,as the period of time after the start of the suspension of the electriccurrent supply increases, cooling proceeds, and it becomes difficult toform a nugget during the subsequent supply of the electric current.

In view of the above, whether a nugget with a large diameter can beformed without the occurrence of expulsion by utilizing the abovephenomenon while the electrode force is controlled was studied. As aresult, it was determined that in an initial stage of the supply ofelectric current, the electric current supply and the suspension of thesupply are repeated, and the electrode force is controlled appropriatelyduring the repetitions. This allows the final diameter of the nugget tobe increased while the occurrence of expulsion is prevented.

Embodiments according to the present invention are as follows.

[1] A resistance spot welding device for welding a sheet set includingat least two overlapping steel sheets by holding the sheet set between apair of welding electrodes and supplying an electric current to thesheet set under pressure, the resistance spot welding device comprising:

a pair of electrodes;

an electrode force gauge that measures an electrode force F applied bythe electrodes; and

a controller that controls an electric current supply to the electrodesaccording to the electrode force F measured by the electrode forcegauge,

wherein the controller controls the electric current supply such that,when the electrode force F measured by the electrode force gauge afterthe electric current supply is started changes from an initial electrodeforce Fi to an electrode force F_(h) ⁽¹⁾ represented by formula (1)while a lapse from the start of the electric current supply is between20 ms and 80 ms inclusive, a suspension of the electric current supplyfor from 20 ms to 60 ms inclusive is started, and then,

the electric current supply is resumed when the electrode force Freaches an electrode force F_(c) ⁽¹⁾ represented by formula (2):1.03×Fi≤F _(h) ⁽¹⁾≤1.15×Fi,  (1)1.01×Fi≤F _(c) ⁽¹⁾≤0.99×F _(h) ⁽¹⁾.  (2)

[2] The resistance spot welding device according to [1], wherein thecontroller further controls the electric current supply such that,

after the suspension of the electric current supply, the electriccurrent supply for from 20 ms to 80 ms inclusive and the suspension ofthe electric current supply for from 20 ms to 60 ms inclusive arerepeated at least once, and that,

when the electrode force F during an Nth electric current supply changesfrom an electrode force F_(c) ^((N-1)) immediately after an (N−1)thsuspension of the electric current supply to an electrode force F_(h)^((N)) represented by formula (3), the suspension of the electriccurrent supply is started, and then,

the electric current supply is resumed when the electrode force Freaches an electrode force F_(c) ^((N)) represented by formula (4):1.04×F _(c) ^((N-1)) ≤F _(h) ^((N))≤1.15×F _(c) ^((N-1)),  (3)F _(c) ^((N-1)) ≤F _(c) ^((N))≤0.99×F _(h) ^((N)),  (4)

where N is a natural number of 2 or more.

[3] The resistance spot welding device according to [1] or [2], whereinthe controller controls the electric current supply such that the periodof a last N+1th repetition of the electric current supply is from 100 msto 300 ms inclusive.

[4] The resistance spot welding device according to any of [1] to [3],further comprising a gun arm to which the pair of electrodes is mounted,

wherein the electrode force gauge is a strain gauge that measures strainof the gun arm.

According to embodiments of the present invention, when a sheet setincluding at least two overlapping steel sheets is subjected toresistance spot welding, a large nugget can be formed while theoccurrence of expulsion is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing the outline of resistance spotwelding.

FIG. 2 is a graph showing changes in electrode force with respect to theinitial electrode force when the resistance spot welding was performedwhile the supply of electric current and the suspension of the supplywere repeated.

FIG. 3 is a graph showing changes in electrode force with respect to theinitial electrode force when the resistance spot welding was performedwhile a constant electric current was supplied.

FIG. 4 is an illustration showing the structure of a resistance spotwelding device according to an embodiment of the present invention.

FIG. 5 is an illustration showing an example of the structure of theresistance spot welding device according to the embodiment of thepresent invention, an electrode force gauge being composed of a straingauge.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will next be described withreference to the accompanying drawings.

FIG. 4 is an illustration showing the structure of a resistance spotwelding device according to the embodiment of the present invention.This device is a device for resistance spot welding in which a sheet set3 including at least two overlapping steel sheets (1 and 2) is heldbetween a pair of electrodes and welded by applying an electric currentunder pressure. This device includes a pair of electrodes (a lowerelectrode 4 and an upper electrode 5), an electrode force gauge 6, and acontroller 7.

The electrode force gauge 6 measures an electrode force F applied by theelectrodes 4 and 5 (hereinafter referred to simply as the electrodeforce F) and outputs the measured electrode force F to the controller 7.For example, the electrode force gauge 6 may be a strain gauge 61 thatmeasures the strain of a gun arm 8 to which the lower electrode 4 andthe upper electrode 5 are mounted, as shown in FIG. 5. When theelectrode force F is applied, a strain approximately proportional to theelectrode force F is generated in the gun arm 8. This strain is measuredby the strain gauge 61 and converted to the electrode force F, and theelectrode force F can thereby be determined.

The electrode force gauge 6 used may be, in addition to the strain gauge61, a load cell capable of measuring the electrode force F, and thestrain gauge 61 and the load cell may be used in combination.

The controller 7 controls the electrode force F according to theelectrode force F measured by the electrode force gauge 6 as follows.Specifically, when the electrode force F after an electric currentsupply is started changes from an initial electrode force Fi to anelectrode force F_(h) ⁽¹⁾ represented by formula (1) while a lapse fromthe start of the electric current supply is between 20 ms and 80 msinclusive, the controller 7 starts a suspension of the electric currentsupply for from 20 ms to 60 ms inclusive.1.03×Fi≤F _(h) ⁽¹⁾≤1.15×Fi  (1)

If F_(h) ⁽¹⁾ is less than 1.03×F_(i), a region in the vicinity of thenugget is not pressurized sufficiently, and the probability of theoccurrence of expulsion becomes high. If F_(h) ⁽¹⁾ is larger than1.15×F_(i), the growth of the nugget is inhibited.

Then the controller 7 controls the electric current supply such that theelectric current supply is resumed when the electrode force F reaches anelectrode force F_(c) ⁽¹⁾ represented by formula (2).1.01×Fi≤F _(c) ⁽¹⁾≤0.99×F _(h) ⁽¹⁾  (2)

If F_(c) ⁽¹⁾ is less than 1.01×Fi, cooling proceeds, and therefore theeffects of the subsequent heating become small. If F_(c) ⁽¹⁾ is largerthan 0.99×F_(h) ⁽¹⁾, the temperature of the nugget is high, andtherefore the possibility of the occurrence of expulsion when theelectric current supply is resumed increases.

Before the electric current is supplied, a pressure is applied for about10 cycles (200 ms) to obtain a stable state, and then the electriccurrent supply is started. The initial electrode force Fi used is theaverage electrode force in the first cycle (20 ms) after the start ofthe supply. The electrode force immediately after the start of theelectric current supply is approximately the same as a set electrodeforce (designated electrode force) of a servo gun, and therefore thedesignated electrode force may be used as the initial electrode forceFi. Alternatively, the average electrode force in a period from 0 ms to20 ms after the start of the electric current supply may be used as theinitial electrode force Fi.

Then the controller 7 controls the electric current supply such that,after the suspension of the electric current supply, the electriccurrent supply for from 20 ms to 80 ms inclusive and the suspension ofthe electric current supply for from 20 ms to 60 ms inclusive arerepeated at least once.

Then, when the electrode force F during the Nth electric current supplychanges from an electrode force F_(c) ^((N-1)) immediately after the(N−1)th suspension of the electric current supply to an electrode forceF_(h) ^((N)) represented by formula (3), the controller 7 starts thesuspension of the electric current supply.1.04×F _(c) ^((N-1)) ≤F _(h) ^((N))≤1.15×F _(c) ^((N-1))  (3)

If F_(h) ^((N)) is less than 1.04×F_(c) ^((N-1)), the probability of theoccurrence of expulsion becomes high. If F_(h) ^((N)) is larger than1.15×F_(c) ^((N-1)), the growth of the nugget is inhibited.

Then the controller 7 controls the electric current supply such that,when the electrode force F reaches an electrode force F_(c) ^((N))represented by formula (4), the electric current supply is resumed. N isa natural number of 2 or more.F _(c) ^((N-1)) ≤F _(c) ^((N))≤0.99×F _(h) ^((N))  (4)

If F_(c) ^((N)) is less than F_(c) ^((N-1)), cooling proceeds, andtherefore the effects of the subsequent heating become small. If F_(c)^((N)) is larger than 0.99×F_(h) ^((N)), the possibility of theoccurrence of expulsion when the electric current supply is resumedincreases.

In embodiments of the present invention, the period of the lastrepetition of the electric current supply is preferably from 100 ms to300 ms inclusive. In the above formulas, the last repetition of theelectric current supply means the (N+1)th repetition of the electriccurrent supply. If the period of the last repetition is less than 100ms, the formation of the nugget is insufficient. If the period of thelast repetition of the electric current supply exceeds 300 ms, theworkability deteriorates, and the contribution of the electric currentsupply to the formation of the nugget is small. The period of the lastrepetition of the electric current supply may be selected optimallywithin the above range according to the period required for the firstelectric current supply and the subsequent repetitions of the electriccurrent supply and the suspension of the electric current supply.

The resistance spot welding device according to embodiments of thepresent invention is not limited to the structures shown in FIGS. 4 and5. It is only necessary for the resistance spot welding device accordingto embodiments of the present invention to include: the vertical pair ofelectrodes which holds parts to be welded and through which theelectrode force and the electric current are applied; and the weldingcurrent controller that can control the welding current freely duringwelding. No particular limitation is imposed on the pressure mechanism(such as an air cylinder or a servo motor), the current controlmechanism (such as an AC or DC current control mechanism), the type (astationary type, a robot gun, etc.), etc.

In certain embodiments, the present invention is applied to a weldingdevice for a sheet set of a plurality of sheets including a galvanizedsteel sheet or a high-strength steel sheet. Galvanized steel sheets andhigh-strength steel sheets are more likely to cause expulsion due to asheet gap than ordinary steel sheets. However, since certain embodimentsof the present invention has the effect of preventing the occurrence ofexpulsion. It is more effective to apply certain embodiments ofinvention to welding of a sheet set including at least one sheetselected from those steel sheets.

Therefore, even when at least one of the steel sheets included in thesheet set to be welded is a high-strength steel sheet having a tensilestrength of 980 MPa or more, the occurrence of expulsion is prevented,and a nugget with a large diameter can be formed.

Even when at least one of the steel sheets included in the sheet set tobe welded is a high-strength steel sheet having a tensile strength of980 MPa or more and containing components including 0.15≤C≤0.30 (% bymass), 1.9≤Mn≤5.0 (% by mass), and 0.2≤Si≤2.0 (% by mass), theoccurrence of expulsion is prevented, and a nugget with a large diametercan be formed.

Moreover, even when at least one of the steel sheets included in thesheet set to be welded is a galvanized steel sheet, the occurrence ofexpulsion is prevented, and a nugget with a large diameter can beformed. The galvanized steel sheet is a steel sheet including a coatinglayer containing Zn as a main component and encompasses anyconventionally known galvanized layer. Specific examples of the coatinglayer containing Zn as a main component include a hot-dip galvanizedlayer, an electrogalvanized layer, an Al coating layer, a Zn—Al coatinglayer, and a Zn—Ni layer.

In the resistance spot welding device according to embodiments of thepresent invention, the electrode force is measured as described above,and the electric current is supplied and suspended while the electrodeforce is controlled appropriately according to the measured electrodeforce during the electric current supply. Thereby, the occurrence ofexpulsion is prevented, and a large nugget can be formed. Therefore,even when work disturbances such as a sheet gap are present, thediameter of the nugget can be ensured stably.

Example 1

In Examples of the present invention, the device shown in FIG. 5 wasused to produce a resistance spot-welded joint using a sheet set 3including two overlapping hot-dip galvannealed steel sheets (a lowersteel sheet 1 and an upper steel sheet 2). Specifically, the device usedin the Examples was a welding device of the C gun type including a servomotor used to press the electrodes. The power source used was a DC powersource.

In this case, the electric current was supplied under conditions shownin Table 1.

The electrodes 4 and 5 used were DR type electrodes made ofalumina-dispersed copper and having a tip radius of curvature R of 40 mmand a tip diameter of 8 mm. The test pieces used were high-strengthsteel sheets having a 980 MPa-class tensile strength and sheetthicknesses of 0.8 to 2.6 mm and a high-strength steel sheet having a1,470 MPa-class tensile strength and a sheet thickness of 2.0 mm. Twosteel sheets of the same type and with the same thickness were stackedand welded.

The electrode force during the electric current supply was measuredusing the strain gauge 61 attached to the C gun. The electric currentsupplied was changed such that the measured electrode force was adjustedto a prescribed value.

Table 1 shows the results of studies on the occurrence of expulsionduring welding and the diameter of the nugget. The diameter of thenugget was evaluated based on the structure of an etched cross sectionas follows. Let the sheet thickness be t (mm). Then a “Good” rating wasgiven when the diameter of the nugget was equal to or larger than 5.5√t. A “Poor” rating was given when the diameter of the nugget was lessthan 5.5 √t. Specifically, a nugget diameter equal to or larger than 5.5√t was set to be an appropriate diameter.

TABLE 1 Test piece Initial First current Second current Evalu- TensileSheet electrode supply First suspension supply Occur- ation of strengththickness force Fi I₁ T₁ F_(h) ⁽¹⁾/ T_(c1) F_(c) ⁽¹⁾/ F_(c) ⁽¹⁾/ I₂ T₂rence of nugget No. (MPa) (mm) (kN) (kA) (ms) F_(i) (ms) F_(i) F_(h) ⁽¹⁾(kA) (ms) expulsion diameter Remarks 1 980 1.2 5.0 10 60 1.05 20 1.030.98 8.5 280 No Good Inventive Example 2 980 1.2 5.0 10 60 1.05 40 1.000.95 8.5 280 Yes Poor Comparative Example 3 980 1.2 5.0 10 60 1.01 201.00 0.99 8.5 280 No Poor Comparative Example 4 980 1.2 5.0 8.5 60 1.010 — — 8.5 220 Yes Poor Comparative Example 5 1470 2.0 6.0 9 60 1.07 201.04 0.98 7.8 300 No Good Inventive Example 6 1470 2.0 6.0 9 60 1.08 601.00 0.92 7.8 320 Yes Poor Comparative Example 7 1470 2.0 6.0 9 60 1.0320 1.00 0.97 7.8 320 No Poor Comparative Example 8 1470 2.0 6.0 8 601.05 0 — — 7.8 260 Yes Poor Comparative Example 9 980 0.8 3.0 10 60 1.1020 1.07 0.97 8.5 300 No Good Inventive Example 10 980 0.8 3.0 10 60 1.100 — — 8.5 300 Yes Poor Comparative Example 11 980 2.0 4.5 10 60 1.11 201.07 0.96 8.5 300 No Good Inventive Example 12 980 2.0 4.5 7.5 80 1.0420 1.02 0.98 8.5 300 No Good Inventive Example 13 980 2.0 4.5 10 60 1.1160 1.02 0.92 8.5 300 No Good Inventive Example 14 980 2.0 4.5 11.5 201.09 20 1.04 0.96 8.5 300 No Good Inventive Example 15 980 2.0 4.5 10 601.11 100 0.98 0.88 8.5 300 Yes Poor Comparative Example 16 980 2.0 4.5 8120 1.16 20 1.13 0.98 8.5 300 No Poor Comparative Example 17 980 2.0 4.58.5 60 1.07 0 — — 8.5 300 Yes Poor Comparative Example 18 980 2.3 5.09.5 60 1.10 20 1.06 0.96 8 300 No Good Inventive Example 19 980 2.3 5.011 20 1.08 20 1.04 0.96 8 300 No Good Inventive Example 20 980 2.3 5.09.5 60 1.10 100 0.98 0.89 8 300 Yes Poor Comparative Example 21 980 2.35.0 7.5 120 1.08 20 1.07 0.99 8 300 Yes Poor Comparative Example 22 9802.3 5.0 8 60 1.06 0 — — 8 260 Yes Poor Comparative Example 23 980 2.66.0 9.5 60 1.08 20 1.05 0.97 8 300 No Good Inventive Example 24 980 2.66.0 8 60 1.08 0 — — 8 260 Yes Poor Comparative Example

In Table 1, I₁ (kA) is the current value in the first electric currentsupply, T₁ (ms) is the current supply time in the first electric currentsupply, and F_(h) ⁽¹⁾/F_(i) is the ratio of the electrode force F_(h)⁽¹⁾ to the initial electrode force Fi. Tc₁ (ms) is the first suspensiontime, and F_(c) ⁽¹⁾/F_(i) is the ratio of the electrode force F_(c) ⁽¹⁾to the initial electrode force Fi. F_(c) ⁽¹⁾/F_(h) ⁽¹⁾ is the ratio ofthe electrode force F_(c) ⁽¹⁾ at which the electric current supply isresumed to the electrode force F_(h) ⁽¹⁾ at which the suspension of theelectric current supply is started. I₂ (kA) is the current value in thesecond electric current supply, and T₂ (ms) is the current supply timein the second electric current supply.

As can be seen from Table 1, when the welding was performed using theresistance spot welding device according to embodiments of the presentinvention, no expulsion occurred, and each nugget formed had anappropriate diameter, in contrast to Comparative Examples.

Example 2

In Examples of the present invention, a servo motor pressurizing-typeresistance welding device attached to a C gun and including a DC powersource was used to perform resistance spot welding on a sheet setincluding three overlapping hot-dip galvannealed steel sheets to therebyproduce a resistance spot-welded joint.

In this case, the electric current was supplied under conditions shownin Table 2.

The electrodes 4 and 5 used were DR type electrodes made ofalumina-dispersed copper and having a tip radius of curvature R of 40 mmand a tip diameter of 8 mm. The test pieces used were 980 MPa classhigh-strength steel sheets having sheet thicknesses of 0.8 to 2.3 mm anda 1,800 MPa class high-strength steel sheet having a sheet thickness of1.2 mm. Three steel sheets of the same type and with the same thicknesswere stacked and welded.

The electrode force during the electric current supply was measuredusing a strain gauge attached to the C gun. The electrode force waschanged such that the measured electrode force was adjusted to aprescribed value.

Table 2 shows the results of studies on the occurrence of expulsionduring welding and the diameter of the nugget. The diameter of thenugget was evaluated based on the structure of an etched cross sectionas follows. Let the sheet thickness be t (mm). Then a “Good” rating wasgiven when the diameter of the nugget was equal to or larger than 5.5√t. A “Poor” rating was given when the diameter of the nugget was lessthan 5.5 √t. Specifically, a nugget diameter equal to or larger than 5.5√t was set to be an appropriate diameter.

The same test was repeated 10 times, and the variations in nuggetdiameter were evaluated. When the diameters obtained were appropriateand the range of variations in nugget diameter was equal to or less than0.1 √t, an “Excellent” rating was given.

TABLE 2 Test piece 1 Initial First current Second current Tensile Sheetelectrode supply First suspension supply strength thickness force Fi I₁T₁ F_(h) ⁽¹⁾/ T_(c1) F_(c) ⁽¹⁾/ F_(c) ⁽¹⁾/ I₂ T₂ No. (MPa) (mm) (kN)(kA) (ms) F_(i) (ms) F_(i) F_(h) ⁽¹⁾ (kA) (ms) 1 980 0.8 3.5 9.5 60 1.1120 1.09 0.97 9   60 2 980 0.8 3.5 9.5 60 1.11 20 1.09 0.97 — — 3 980 0.83.5 9.5 60 1.11 20 1.09 0.97 6   120  4 980 0.8 3.5 8 60 1.08 0 — — — —5 980 2.3 6.0 9 60 1.05 20 1.03 0.98 8.5 60 6 980 2.3 6.0 9 60 1.05 201.03 0.98 — — 7 980 2.3 6.0 9 60 1.05 20 1.03 0.98 6.5 120  8 980 2.36.0 9 60 1.05 0 — — — — 9 1800 1.2 5.5 9 60 1.07 20 1.05 0.98 8.5 60 101800 1.2 5.5 9 60 1.07 20 1.05 0.98 — — 11 1800 1.2 5.5 8 60 1.07 0 — —— — Third current Evalu- Second current Second suspension supply Occur-ation of supply T_(c2) F_(c) ⁽²⁾/ F_(c) ⁽²⁾/ I₃ T₃ rence of nugget No.F_(h) ⁽²⁾/F_(c) ⁽¹⁾ (ms) F_(c) ⁽¹⁾ F_(h) ⁽²⁾ (kA) (ms) expulsiondiameter Remarks 1 1.08 20 1.05 0.98 8 240 No Excellent InventiveExample 2 — — — — 8 240 No Good Inventive Example 3 1.03 20 1.01 0.99 8240 Yes Good Inventive Example 4 — — — — 8 240 Yes Poor ComparativeExample 5 1.05 20 1.03 0.98 8 260 No Excellent Inventive Example 6 — — —— 8 260 No Good Inventive Example 7 1.03 20 1.02 0.98 8 260 No GoodInventive Example 8 — — — — 8 260 Yes Poor Comparative Example 9 1.05 201.03 0.98 8 280 No Excellent Inventive Example 10 — — — — 8 280 No GoodInventive Example 11 — — — — 8 280 Yes Poor Comparative Example

In Table 2, I₁ (kA) is the current value in the first electric currentsupply, T₁ (ms) is the current supply time in the first electric currentsupply, and F_(h) ⁽¹⁾/F_(i) is the ratio of the electrode force F_(h)⁽¹⁾ to the initial electrode force Fi. Tc₁ (ms) is the first suspensiontime, and F_(c) ⁽¹⁾/F_(i) is the ratio of the electrode force F_(c) ⁽¹⁾to the initial electrode force Fi. F_(c) ⁽¹⁾/F_(h) ⁽¹⁾ is the ratio ofthe electrode force at which the electric current supply is resumed tothe electrode force at which the suspension of the electric currentsupply is started. Similarly, F_(h) ⁽²⁾/F_(c) ⁽¹⁾ is the ratio of theelectrode force at which, after the second electric current supply, thesuspension of the electric current supply is started to the electrodeforce immediately after the first suspension. F_(c) ⁽²⁾/F_(c) ⁽¹⁾ is theratio of the electrode force immediately after the second suspension tothe electrode force immediately after the first suspension, and F_(c)⁽²⁾/F_(h) ⁽²⁾ is the ratio of the electrode force immediately after thesecond suspension to the electrode force at which, after the secondelectric current supply, the suspension of the electric current supplyis started. I₂ (kA) and I₃ (kA) are the current values in the second andthird electric current supplies, respectively, T₂ (ms) and T₃ (ms) arethe current supply times in the second and third electric currentsupplies, respectively, and Tc₂ (ms) is the second suspension time.

As can be seen from Table 2, when the resistance spot welding wasperformed according to embodiments of the present invention, noexpulsion occurred, and each nugget formed had an appropriate diameter,in contrast to Comparative Examples. As can also be seen, when thesecond supply of the electric current was performed under the conditionsof embodiments of the present invention, the effect of stabilizing thenugget diameter was obtained, in contrast to other cases.

REFERENCE SIGNS LIST

-   -   1 lower steel sheet    -   2 upper steel sheet    -   3 sheet set    -   4 lower electrode    -   5 upper electrode    -   6 electrode force gauge    -   61 strain gauge    -   7 controller    -   8 gun arm    -   9 nugget

The invention claimed is:
 1. A resistance spot welding device forwelding a sheet set including at least two overlapping steel sheets byholding the sheet set between a pair of electrodes for welding andsupplying an electric current to the sheet set under pressure, theresistance spot welding device comprising: the pair of electrodes; anelectrode force gauge that measures an electrode force F applied by theelectrodes; and a controller that controls a supply of the electriccurrent to the electrodes according to the electrode force F measured bythe electrode force gauge, wherein the controller controls the electriccurrent supply such that, when the electrode force F measured by theelectrode force gauge after the electric current supply is startedchanges from an initial electrode force Fi to an electrode force F_(h)⁽¹⁾ represented by formula (1)1.03×Fi≤F _(h) ⁽¹⁾≤1.15×Fi,  (1) while a lapse from the start of theelectric current supply is between 20 ms and 80 ms inclusive, asuspension of the electric current supply of from 20 ms to 60 msinclusive is started, and then, the electric current supply is resumedwhen the electrode force F reaches an electrode force F_(c) ⁽¹⁾represented by formula (2):1.01×Fi≤F _(c) ⁽¹⁾≤0.99×F _(h) ⁽¹⁾  (2).
 2. The resistance spot weldingdevice according to claim 1, wherein the controller further controls theelectric current supply such that, after the suspension of the electriccurrent supply, the electric current supply of from 20 ms to 80 msinclusive and the suspension of the electric current supply of from 20ms to 60 ms inclusive are repeated at least once, and that, when theelectrode force F during an Nth electric current supply changes from anelectrode force F_(c) ^((N-1)) immediately after an (N−1)th suspensionof the electric current supply to an electrode force F_(h) ^((N))represented by formula (3),1.04×F _(c) ^((N-1)) ≤F _(h) ^((N))≤1.15×F _(c) ^((N-1)),  (3) thesuspension of the electric current supply is started, and then, theelectric current supply is resumed when the electrode force F reaches anelectrode force F_(c) ^((N)) represented by formula (4):F _(c) ^((N-1)) ≤F _(c) ^((N))≤0.99×F _(h) ^((N)),  (4) wherein N is anatural number of 2 or more.
 3. The resistance spot welding deviceaccording to claim 1, wherein the controller controls the electriccurrent supply such that a period of a last (N+1)th repetition of theelectric current supply is from 100 ms to 300 ms inclusive.
 4. Theresistance spot welding device according to claim 1, further comprisinga gun arm to which the pair of electrodes is mounted, wherein theelectrode force gauge is a strain gauge that measures strain of the gunarm.
 5. The resistance spot welding device according to claim 2, whereinthe controller controls the electric current supply such that the periodof a last (N+1)th repetition of the electric current supply is from 100ms to 300 ms inclusive.
 6. The resistance spot welding device accordingto claim 2, further comprising a gun arm to which the pair of electrodesis mounted, wherein the electrode force gauge is a strain gauge thatmeasures strain of the gun arm.
 7. The resistance spot welding deviceaccording to claim 3, further comprising a gun arm to which the pair ofelectrodes is mounted, wherein the electrode force gauge is a straingauge that measures strain of the gun arm.
 8. The resistance spotwelding device according to claim 5, further comprising a gun arm towhich the pair of electrodes is mounted, wherein the electrode forcegauge is a strain gauge that measures strain of the gun arm.